DOC. 313 MARCH 1917 303 313. From Willem de Sitter Leyden, 20 March 1917 Dear Mr. Einstein, I have found that the equations Guv =Ag fLV 0, thus your equations (13a) without matter,[1] can be satisfied by the guv’s, which are given by[2] ds2 -fir2 - dy2 - dz2 + c2dt2 (1 - /ah2)2 (1) A h2 = c2t2 - x2 - y2 - z2. x, y, z, t can become oo. At infinity (either spatial or temporal or both) the guv’s become 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Here we thus have a system of integration constants, or boundary values at infinity, which is invariant under all transformations. For relatively small values of h, i.e., in our spatial and temporal proximity, we have the guv’s of the old theory of relativity if only u(A) is small enough. This is achieved without supernatural masses only through the introduction of the undetermined and undeterminable constant A in the field equations.[3] I do not know if it can be said that “inertia is explained” in this way. I do not concern myself with explanations.[4] If a single test particle existed in the world, that is, there were no sun and stars, etc., it would have inertia. Also in your theory, as I see it, if physical masses (sun, etc.) were not there. Conjecturing that supernatural masses did not exist is just as impossible in your theory as saying “assume the world did not exist.”[5] (1) can also be interpreted in such a way that the four-dimensional world is finite, with a radius given by A = 3/R2. The analogy with your solutions emerges from the following comparison:[6]